1. Field of the Invention
The present invention relates to an improved method for the treatment of waste water such as industrial waste water or domestic waste water, by an activated sludge process, where the organic substances contained in the waste water are decomposed with aerobic microorganisms in order to purify the waste water.
2. Description of the Background
Various means of purifying waste water with aerobic microorganisms have been developed in the past and put to practical use, including the so-called activated sludge process. In the conventional apparatus for purification of waste water, the aeration tank where organic substances are decomposed with microorganisms by vigorous aeration does not contain partitions, and aeration is effected uniformly in the aeration tank. In such a case, the pH value of the waste water which is placed in the aeration tank is generally 7 or so, and it has been said that the pH value in the inside of the aeration tank should preferably be slightly alkaline, e.g., from 7 to 8 or so.
On the other hand, various methods of using plural aeration tanks for various purposes have been developed. (For example, refer to Japanese Patent Application Laid-Open Nos. 54-77461, 58-98189, 59-39391, 60-19097 and 62-1496 and J. Ferment. Technol., Vol. 63, No. 4, pages 357 to 362, 1985.)
The present inventors had previously developed an activated sludge process for the treatment of waste water, wherein the aeration tank unit in the activated sludge processing equipment is actually comprised of three tanks of a first tank, a second tank and a third tank. In the process, waste water and microorganisms are fed into the first tank, with the ratio of aeration in the first tank ranging from 0.5 to 0.8 by volume/minute relative to the amount of liquid in the first tank. In the second tank, the ratio of aeration ranges from 0.1 to 0.4 by volume/minute relative to the liquid amount in the second tank, while the ratio of aeration in the third tank ranges from 0.1 to 0.3 by volume/minute relative to the liquid amount in the third tank (Japanese Patent Application Laid-Open No. 1-199694).
On the other hand, various methods using ozone have also been developed in the activated sludge process (Japanese patent Application Laid-Open Nos. 51-76859, 53-32967, 55-27072, 57-122998 and 57-153797).
Among them, the method disclosed in Japanese Patent Application Laid-Open No. 57-122998 is directed to rendering waste ozone gas innoxious by feeding waste ozone gas, which is obtained mainly during the sophisticated treatment of water with activated sludge, into an aeration tank and at the same time improving the sedimentation property of activated sludge. The waste ozone gas of the process ranges in an amount from 0.5 to 3 g per Nm.sup.3 of an aeration volume and the operation is carried out by adding 5.times.10.sup.-3 to 5.times.10.sup.-1 g/hour of ozone per g hour of activated sludge.
It is known that in the activated sludge process for treating waste water using three aeration tanks, waste water can be treated in a high concentration and high load. However, the method has a problem common to the activated sludge process that bulking occurs during the operation, depending on the kind of waste water employed and other factors. Accordingly, a need continues to exist for an improved method of treating waste water in a three tank aeration system.